Coilable dual wall corrugated pipe and related method

ABSTRACT

High flow capacity corrugated coilable dual wall plastic pipe having improved longitudinal flexibility and a bend radius sufficient to permit coiling and plowing of such pipe into the ground using automated installation equipment. The coilable dual wall pipe has an outer corrugated wall formed of a thermoplastic material such as high density polyethylene, and an inner smooth wall formed of a different more flexible and resilient thermo-bondable material that has an enhanced strain capacity and lower modulus of elasticity. The inner wall material also has enhanced mechanical properties which generally improve processing and help prevent excessive drag and tearing of the liner material during the manufacturing process. Suitable material blends for the inner wall in corporating a thermoplastic elastomer additive in combination with a thermoplastic material have proven suitable in providing the required longitudinal flexibility and pipe bend radius to permit the desired pipe coiling and plow installation without failure.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is an application for a patent which is also disclosedin Provisional Application Ser. No. 61/864,898, filed on Aug. 12, 2013,entitled “Coilable Dual Wall Pipe and Related method,” the benefit ofthe filing date of which is hereby claimed.

FIELD OF INVENTION

The present disclosure relates generally to the field of coilablecorrugated plastic drainage pipe, and more particularly to a coilableplastic drainage pipe of dual wall construction having an outercorrugated wall with an inner smooth liner wall formed integrallytherewith.

BACKGROUND

The statements in this section merely provide background informationrelated to the present disclosure and may not constitute prior art.

Single wall corrugated high density polyethylene (HDPE) pipe was firstdeveloped and has been in existence since the 1970's. Such single wallplastic pipe was first formed in small diameters (e.g., 3″ diameter) asa cost effective and productivity enhancing replacement for clay tileused in agricultural applications. Single wall corrugated HDPE pipeallows for significant installation efficiencies due to its flexiblenature and the fact that it is available in long and coiled lengths. Thelongitudinal flexibility and coilable nature of the pipe permitssubstantially automated plowing of long lengths of the pipe into theground, as opposed to installing individual sections of pipe throughopen trench excavation.

However, single wall corrugated pipe does have its drawbacks. Forinstance, single wall corrugated HDPE is not hydraulically efficientrelative to other non-coilable pipe. For gravity flow applications, theinterior roughness of pipe is measured by the Mannings coefficient.Single wall corrugated HDPE has a relatively rough interior surface,with a minimum Manning's coefficient falling in the range of0.021-0.030. Moreover, as technology for installing single wallcorrugated HDPE has progressed, so have the number of field failures forcorrugated HDPE. The newer installation technology for single wallagriculture applications involves plowing the pipe in the ground.However, with the newer technology, contractors have the tendency toaxially stretch the pipe, which results in a reduction in structuralproperties. The axial stretching of the pipe has led to numerous fieldfailures.

As the corrugated HDPE industry progressed, the diameter ranges ofcorrugated single wall pipe increased from 3″ up to 24″, allowing forsimilar efficient installation practices for larger diameter pipe.Additionally, with further advancements in manufacturing technology,dual wall corrugated HDPE pipe was soon developed. Dual wall pipe hasthe same corrugated exterior but a smooth wall interior, resulting indramatic improvements in fluid flow capacity and performance. Dual wallcorrugated HDPE has a typical Manning's coefficient falling in the rangeof 0.010-0.015, which makes it more hydraulically efficient than singlewall corrugated HDPE. Additionally, dual wall corrugated HDPE's smoothinterior is less likely to get clogged with silts and sands that are inthe water being transported in the pipe. Dual wall corrugated HDPE pipewas first introduced into the market on or about in the mid 1980's.

Corrugated dual wall HDPE pipe exhibits many of the same characteristicsas single wall HDPE pipe, such as strength and lightweight construction,but also offers significantly increased flow capacity due to the smoothinner wall. Moreover, when tested for axial pipe stiffness in accordancewith ASTM Standards (i.e., ASTM F405), the contents of which areincorporated herein by reference, corrugated dual wall pipe has superiorstrength. Corrugated dual wall HDPE has sufficient axial strength toresist stretching the pipe in an axial direction, thereby preserving itsstrength when mis-installed by contractors. By contrast, conventionalcurrent-day single wall corrugated HDPE pipe does not have sufficientaxial strength to keep from being stretched during the installationprocess.

In addition to the above, with higher axial stiffness, corrugated dualwall HDPE pipe has allowed for the development of a bell and spigot typecoupling system. Increased axial stiffness associated with the innerliner has enabled the assembly of a bell and spigot pipe configurationutilizing a compression fit. The compression fit caused by an O-ringstyle gasket is useful in keeping silts out of the pipe. Such acompression fit typically requires about two lbs. per inch of nominaldiameter (2.0 lb/in dia.) axial compression force to engage a bell andspigot coupling system. Corrugated single wall HDPE pipe, on the otherhand, utilizes a split coupler that wraps around the outside of thepipe, or an internal snap coupler. This is necessary because the axialstiffness of corrugated single wall pipe is insufficient to permit thecompression fit associated with the bell and spigot type couplingsystem. Both corrugated single wall HDPE pipe joints are consideredinferior to the bell and spigot coupling system used with dual wallcorrugated HDPE pipe.

The drawback with HDPE dual wall pipe, however, which persists to thisday, is the longitudinal stiffness of such pipe caused by the presenceof the smooth inner liner wall and general inelasticity of the material;its inability to flex longitudinally prevents many of the installationefficiencies provided by the single wall pipe design. Conventionalcurrent-day corrugated HDPE dual wall pipe cannot be coiled or flexedlongitudinally without breakage. Consequently, this has required asignificant change in installation practice from plowing pipe into theground to open trench excavations, a practice which is far lessefficient and significantly more costly.

Open trench excavation may be appropriate and accepted for certain civilconstruction applications due to the high expectations for installedperformance and the relatively small amount of pipe required to beinstalled on any single project. For agricultural applications, however,along with flow capacity, installation efficiency is the primaryconcern. Usually, there are only narrow windows of time between springthaw and spring planting, and between harvest and ground freezing,during which installation is reasonably practicable, and most projectsrequire installation of thousands and tens of thousands of feet of pipe.Consequently, for agricultural projects, improving installation time andlowering installation cost has a significant impact on the overallproject cost. The ability to install pipe via the use of automatedplowing equipment is paramount, and the use of pipe having increasedaxial strength to prevent field failures would also help significantlyto increase installation production rates. Because of this installationcost component, there has been a long-felt, strong and unsatisfied needfor innovation in the area of high flow capacity corrugated HDPE pipe(i.e., corrugated dual wall pipe) which is flexible enough to be plowedinto the ground, has sufficient axial strength to be plowed at higherrates, and is also flexible enough to be coiled.

Finding a solution to the foregoing problems, however, is furthercomplicated by the fact that numerous other variables, such asinstallation temperatures, processing conditions, pipe diameter, pipeprofile geometry, etc., have an effect on and may determine the needs ofa particular application. The specific composition of material utilizedin the construction of coilable corrugated dual wall pipe for one set ofcircumstances or application may vary dramatically from that of another.Therefore, the appropriate solution requires suitable versatility toaccommodate variations for differing application requirements.

SUMMARY

According to various aspects of the present disclosure, exemplaryembodiments are provided herein of an improved high flow capacitycorrugated coilable dual wall plastic pipe and processes ofmanufacturing same. In order to achieve this objective, the material ofthe inner liner wall requires modification to promote flexibility andresiliency, and facilitate coiling of the pipe and the ability to plowthe dual wall pipe into the ground without failure. Accordingly, fortypical HDPE dual wall pipe, the inner liner wall needs to be formed ofa different HDPE-bondable material having an enhanced strain capacityand reduced modulus of elasticity. Additionally, the liner materialshould preferably have an enhanced melt strength and relatively lowcoefficient of friction to help prevent sticking and tearing of theliner material during the manufacturing process.

The use of linear low density polyethylene (LLDPE) as all or part of theinner liner material has been contemplated as one means of lowering themodulus of elasticity and enhancing the flexibility of the inner wallmaterial. The LLDPE material may be used either at 100% loading or bemixed with HDPE at a percentage where the LLDPE is the majoritycomponent. However, if LLDPE is used for the inner liner wall,processing alterations may need to be made by increasing the taper ofthe corrugator cooling mandrel to offset material drag, since LLDPEtends to exhibit a higher coefficient of friction and lower meltstrength than HDPE. Moreover, LLDPE is relatively costly to use;therefore, given the challenges faced with production start-up andproduction consistency, this approach may be less desirable.

As a preferred alternative approach, an appropriate additive may be usedto form a blend material for the inner wall that exhibits propertiessimilar to that of LLDPE. Such blended material would be thermallybondable with the outer corrugated wall, but would have enhancedelastomeric properties that provide greater flexibility and resiliencyto the pipe in general. One additive contemplated is the use of athermoplastic elastomer (TPE). TPE has a greater strain capacity andlower modulus of elasticity than HDPE and, in appropriate proportions,can provide the desired enhancement in elastomeric properties of thesmooth inner wall. Blend optimization is critical and may vary dependingon a number of different factors, including without limitation pipeprofile design, manufacturing equipment, processing speed, and/orprocessing conditions. Additionally, for the final application of theproduct, a proper balance of flexibility for coiling the pipe andproviding sufficient axial strength to prevent the installation relatedchanges to the structural properties of the pipe are necessary in thefinal optimized blend of materials. Lastly, the TPE blend formulationexhibits the ability to rebound after coiling to form a smootherinterior surface, thereby reducing the Mannings coefficient to a valueless than 0.021 which is the minimum Mannings coefficient used forcorrugated single wall HDPE. Using such a TPE additive has also beenfound to exhibit other improved mechanical properties that furtherimproves processability of the pipe.

An inner wall inspection system and method is also provided which helpsminimize potential disruptions in the extrusion process and alleviateconcerns as to the improper formation of the inner wall. The inspectionsystem utilizes automated laser technology that is telescopicallyreceived within the dual wall pipe to scan the surface of the inner wallfor defects. Multiple cameras function to capture 360° coverage of theinner wall annular surface and report any noted defects to thecorrugator operator.

Further areas of applicability will become apparent from the detaileddescription provided herein. It should be understood that thedescription and specific examples are intended for purposes ofillustration only and are not intended to limit the scope of the presentdisclosure.

DRAWINGS

These and other objects and advantages of the invention will more fullyappear from the following description, made in connection with theaccompanying drawings, wherein like reference characters refer to thesame or similar parts throughout the several views, and in which:

FIG. 1 is a side elevation view of a section of conventional dual wallpipe of bell and spigot design, with a portion thereof broken away toshow the dual wall construction of the pipe;

FIG. 2 is a side elevation view of a section of coilable dual wall pipeconstructed in accordance with and embodying the principles of theinvention described herein, with a portion thereof broken away to showthe flexibility and construction of the pipe;

FIG. 3 is a side elevation view of an indeterminate length of coilabledual wall pipe constructed in accordance with and embodying theprinciples of the invention described herein;

FIG. 4 is a diagrammatic side elevation showing the manner of use of athree dimensional laser imager for inspecting the integrity of the innerwall of a section of coilable dual wall pipe;

FIG. 5 is a graphical representation of a recorded output from one ofthe laser imager cameras, showing a display of the inner wall of a dualwall pipe having defects; and

FIG. 6 is a graphical representation of a recorded output from one ofthe laser imager cameras, showing a display of the inner wall of a dualwall pipe having no defects.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is notintended to limit the present disclosure, application, or uses. Itshould be understood that throughout the drawings, correspondingreference numerals indicate like or corresponding parts and features.

With reference now to the drawings, and particularly to FIG. 1, anexemplary embodiment is shown of a typical section of corrugated dualwall pipe 1 as conventionally constructed. For agricultural purposes,such corrugated dual wall pipe is typically constructed of high densitypolyethylene (HDPE), but the use of other suitably strong thermoplasticmaterials, including without limitation polypropylene (PP) andpolyvinylchloride (PVC), are also contemplated and considered to bewithin the scope of the present invention. For convenience, however, thefollowing discussion will focus on pipe formed primarily of HDPE, itbeing understood that the same principles will apply equally to pipeformed of other thermoplastic materials.

Therefore, as shown in FIG. 1, the section of dual wall pipe 1 has anouter corrugated wall 3 formed of HDPE. The outer corrugated wall 3 isdefined by spaced apart successive annular corrugations 5 with annularvalley-defining portions 7 deposed therebetween. Each successivecorrugation 5 has an outer crest portion 9 with annular sidewalls 11extending radially inward therefrom. The sidewalls 11 of eachcorrugation 5 interconnect with and define the adjacent valley-definingportions 7 of the corrugated wall 3. The interior of each section ofdual wall pipe 1 is then defined by an inner cylindrical smooth linerwall 13 which is also formed of HDPE. The inner wall 13 is attached toand integrally formed with the root 15 of each of the valley-definingportions 7 of the outer corrugated wall 3 in a manner well known in theart.

The section of dual wall pipe 1 disclosed in FIG. 1 is designed with aconventional bell and spigot coupling system. As shown, one end of pipe1 constitutes a male spigot 17 and the opposite end comprises anintegral female coupling element or bell coupler 19. In a manner alsowell known in the art, each bell coupler 19 is adapted to receive thespigot end 17 of a similarly constructed section of pipe 1 in end-to-endfashion to form a corrugated piping system of desired length. Dueprincipally to the axial stiffness of such HDPE dual wall pipe, the belland spigot construction has become the preferred industry standard forinstallation via methods of open trench excavation.

As noted previously, installation efficiencies are not fully realizedwith today's corrugated single wall HDPE pipe because of axialstretching associated with higher installation rates. However, there aresignificant drawbacks to using conventional HDPE dual wall pipe.Particularly in agricultural applications requiring installation ofthousands and tens of thousands of feet of pipe, the longitudinalstiffness of such pipe prevents coiling and plowing the pipe into theground like flexible single walled pipe. Consequently, many of theinstallation efficiencies provided by corrugated single wall pipe arelost, and conventional HDPE dual wall pipe is limited to piece-by-pieceinstallation using methods of open trench excavation. Installation inthis manner is far less efficient and significantly more costly thanplowing the pipe into the ground using substantially automatedinstallation equipment.

In light of the foregoing limitations of HDPE corrugated dual wall pipe,producing an improved high flow capacity coilable dual wall pipe whichexhibits the longitudinal flexible nature of single wall corrugatedplastic pipe and the axial strength similar to dual wall corrugatedplastic pipe requires a different approach to the process and product.First, in order to promote field flexibility without failure, the innerliner wall of the pipe must be manufactured with a material other than100% HDPE. Conventional HDPE and other thermoplastic materials haveproven too inflexible to facilitate coiling and plow installationwithout failure. In order to promote improved overall flexibility andresiliency of the inner liner wall, the liner material should preferablyhave an enhanced strain capacity and reduced modulus of elasticity. Suchflexibility and resiliency is a matter of paramount importance in thatthe resulting pipe must be capable of achieving significant bending andflexing without cracking, splitting or other failure when coiling thepipe or installing the pipe into the ground using automated plowingequipment. Additionally, the inner wall material must have sufficientstrength to allow axial stiffness in tension to resist stretching duringhigh rates of plowing the pipe and sufficient axial stiffness incompression to enable the assembly of bell and spigot joints (i.e., atleast approximately 2.0 lb/in nominal diameter). The balance betweenflexibility to coil the pipe and axial stiffness for high rateinstallations and bell and spigot assembly makes this invention unique.

While more flexibility and resiliency is critical, other manufacturingparameters must also be achieved. For instance, the inner wall materialmust have sufficient melt strength to help avoid tearing of the materialduring the manufacturing process. The material coefficient of frictionis also important. It is preferable that the liner material have arelatively low coefficient of friction at the liner melt temperature soas to further prevent sticking of the material to the cooling mandreland tearing of the liner during the manufacturing process. Moreover,while achieving the above criteria, the liner material must still besuitably compatible for bonding with a thermoplastic material, since itis intended that the outer corrugated wall of the pipe still be formedof HDPE or other thermoplastic material.

As noted above, the liner must be able to bend without tearing and relaxto its original shape after bending. Furthermore the liner must be ableto provide sufficient axial strength to avoid overstretching during highspeed installation processes as well as excessive compression duringbell and spigot assembly. Taking all of the above considerations intoaccount, it is contemplated that the resulting corrugated dual wall pipeshould have a high degree of longitudinal flexibility such that it iscapable of being coiled without damage to a minimum bend radius “R” thatis approximately one-half (0.5) the pipe nominal diameter, with apreferred target bend radius falling within the approximate range of 0.5to 4.0 times the pipe nominal diameter. Moreover, such coilable dualwall pipe should have sufficient axial strength to withstand an axialcompressive force of at least about two pounds per inch nominal pipediameter (2.0 lb/in dia.), which is the axial force typicallyexperienced during assembly of a bell and spigot compression fitcoupling system.

With the above in mind, according to various aspects of the presentdisclosure, and with specific reference to FIGS. 2 and 3, exemplaryembodiments are provided herein of an improved high flow capacitycorrugated coilable dual wall plastic pipe 101 and processes ofmanufacturing same. In one exemplary embodiment, the use of linear lowdensity polyethylene (LLDPE) is contemplated to lower the modulus ofelasticity and enhance the flexibility of the liner material. While notintending to be limiting, it has been found that maintaining the shortterm modulus of elasticity below approximately 80,000 psi (pounds persquare inch) has been advantageous in promoting the desired elasticityin the liner material. However, it should be understood that the optimumvalue or range for the modulus of elasticity of the liner material couldvary depending on a number of other factors including, withoutlimitation, the pipe profile design, installation techniques, pipe jointconfiguration, manufacturing equipment and/or processingconditions/speed.

In order to meet the desired bend radius and axial strength requirementsof the resultant coilable dual wall pipe, if LLDPE is used as acomponent of the liner material, it must be used either at 100% loadingor be mixed with HDPE at a percentage where the LLDPE is the majoritycomponent. Here again, although external factors may impact the optimumblend, it has been found that the use of LLDPE in amounts greater thanabout 60% loading is generally preferred.

Regardless of blend optimization, when LLDPE is used for the liner,there tend to be challenges with production start-up and productionconsistency. It has been found that the low melt strength of the innerliner wall tends to increase to a level which can potentially createexcessive drag on the cooling mandrel during manufacturing. As notedpreviously, such increased drag is undesirable in that it can cause thepipe liner to tear and/or the pipe corrugator to stop. Therefore, ifLLDPE is to be used all or in part as the liner material, it may benecessary to somehow enhance the melt strength properties as themanufactured pipe travels through the pipe corrugator. One meanscontemplated for dealing with the low melt strength is to properly taperthe cooling mandrel of the corrugator. This tends to help reduce theprocessing issues of the liner therewith while maintaining the necessarycontact in order to adequately cool the liner.

LLDPE is a relatively expensive material, however, and due to the heavyloading required for effective results, the use of such material as allor a significant part of the inner liner wall can be costly. Therefore,although the use of LLDPE does provide a workable solution, given theincreased cost involved and the challenges faced with productionstart-up and production consistency, more cost competitive solutions mayexist, thus making this approach be less desirable.

As an alternative approach, an appropriate additive may be used withHDPE or other thermoplastic materials to form a bondable blend materialfor the inner wall that exhibits enhanced elastomeric and mechanicalproperties. For instance, the use of a thermoplastic elastomer (TPE) isone suitable additive to improve the elastomeric properties of the innerwall and provide greater flexibility and resiliency to the pipe. TPE'sare generally low modulus, flexible, thermo-bondable materials which canbe stretched repeatedly to more than twice their original length withthe ability to return to nearly the original length. TPEs perform muchmore like an elastomer, versus the relatively “rigid” behavior of athermoplastic. Thermoplastics, on the other hand, have a much highermodulus of elasticity and exhibit a much lower tolerance tostrain/stretching, i.e., thermoplastics will become permanently deformedat a much lower strain level than TPE's. Where TPE's can tolerate 50%strain with the ability to return to the original shape or up to 200%with minimal permanent deformation, thermoplastics can typicallytolerate less than 10% strain without causing permanent deformation.Given the greater strain capacity and lower modulus of elasticity of aTPE, in appropriate proportions, it can provide the desired enhancementto the elastomeric properties of the smooth inner wall.

Vistamaxx® (an Exxon Mobil product) is one such TPE that is contemplatedas a potentially viable additive for enhancing the elastomericproperties of the inner wall of a dual wall pipe. Vistamaxx® is apolypropylene based thermoplastic elastomer. It has been found that whenVistamaxx® is mixed in appropriate amounts with HDPE, medium density,low density or linear low density polyethylene, the Vistamaxx® additiveallows the finished blend to exhibit similar mechanical properties toLLDPE, thereby producing an improved flexible liner wall with enhancedstrain capacity and reduced modulus of elasticity.

In addition to the above, as compared with LLDPE, it has been found thatuse of certain TPE additives blended with a thermoplastic material haveimproved mechanical properties which help improve processing of the dualwall pipe during manufacturing, thus avoiding some of the potentialpitfalls and/or inconsistencies involved with using LLDPE. For instance,it is believed that the use of certain TPE additives may have the addedbenefit of enhancing the material blend melt strength of the linermaterial. Improving the material melt strength helps lessen the tendencyfor the material to tear when in a molten state during manufacturing,thus further improving processability of the pipe.

Various liner material formulations have been developed using a TPEadditive with extremely promising results. The ratio of TPEadditive/carrier resin is critical to achieving maximum processingefficiency and field performance. While blend optimization may vary anddepend on a number of different factors, it has been found that a blendof less than approximately 40% TPE additive by weight and greater thanapproximately 60% thermoplastic material exhibits the most promisingresults. Factors that may have an effect on the optimum blend include,but are not limited to, the pipe profile design, installation practicesand temperature, manufacturing equipment, processing speed, and/orprocessing conditions.

For purposes of illustration, shown in FIGS. 2 and 3 of the drawings isan exemplary embodiment of an improved high flow capacity corrugatedcoilable dual wall plastic pipe 101 constructed in accordance with thepresent invention. Such dual wall pipe 101 is characterized in having anouter corrugated wall 103 formed of HDPE or other suitable thermoplasticmaterial, and a smooth cylindrical inner liner wall 105 formed of adifferent material meeting the parameters of the embodiments discussedabove. As shown by the dashed representation of the coilable dual wallpipe 101 in FIG. 2, with the increased elasticity and strain capacity ofinner wall 105, pipe 101 exhibits substantially improved longitudinalflexibility, and is capable of being bent and coiled for ease oftransportation and automated installation. FIG. 2 also shows the innerwall 105 of the pipe 101, which is essentially smooth. This inner wall105 improves the hydraulic characteristics of the pipe 101. Other thanthe enhanced flexibility and resiliency of the corrugated dual wall pipe101, in most other respects such pipe is constructed similar to theconventional pipe 1, shown in FIG. 1. Notably, however, as best depictedin FIG. 3, the longitudinally flexible coilable dual wall pipe 101 maybe produced in long indeterminate lengths, as opposed to numerousindividual sections of conventional dual wall pipe 1, each section ofwhich requires some form of coupling system (e.g., bell & spigot) forjoining the same end-to-end.

More importantly, the inner liner wall 105 of dual wall pipe 101 hasbeen produced with an enhanced strain capacity and modulus of elasticitywhich promotes sufficient flexibility and resiliency to facilitatebending of the pipe 101 to a minimum bend radius “R” that isapproximately one-half (0.5) the pipe nominal diameter, with a preferredtarget bend radius falling within the approximate range of 0.5 to 4.0times the pipe nominal diameter. With such enhanced longitudinalflexibility, as shown in FIGS. 2 and 3, coiling of the improved dualwall pipe 101 is now possible, and installation by plowing the pipe intothe ground in a manner similar to single wall corrugated pipe is nowavailable.

Furthermore, the pipe 101 with the inner wall 105 has improved axialstrength to allow plowing at increased rates. When tested in accordancewith the applicable ASTM Standard F405, the axial pipe stiffness of thecoilable dual wall pipe 101 is greater than that of conventional singlewall corrugated HDPE pipe. From a manufacturing standpoint, there arevirtually no required modifications other than to account for thepossibility of slightly increased processing friction caused by themodified liner material. Accordingly, an improved high flow capacitycoilable dual wall corrugated pipe having all the installationefficiencies equal to or great than conventional single wall corrugatedplastic pipe is now available.

The manufacturing process for dual wall pipe generally involves the useof a co-extrusion process where extrusion dies are fed polymer melt froman extruder or plurality of extruders. The extrusion dies form thepolymer melt into inner and outer polymer melt parisons. The outer meltparison exits one extrusion die orifice into a series of vacuum moldblocks run on a continuous corrugator, where the parison is thermoformedto create the outer corrugated wall. The inner melt parison exitsanother extrusion die orifice and typically passes over a cooling andsizing mandrel, where it becomes thermo-bonded to the valley portions ofthe corrugated outer wall, thereby forming the smooth inner wall of thedual wall corrugated plastic pipe.

Thermoplastic poly-olefins such as high density polyethylene,polypropylene, polyvinylchloride and blends or mixtures thereof may beutilized to manufacture dual wall corrugated plastic pipe. Even when thepolymer melts for each wall is consistent in material composition,variations between material lots and extrusion processing can presentdifficulties in maintaining uniformity of wall thickness and linearweight of the dual wall plastic pipe. Variations in viscosity andelasticity can cause the extruded pipe walls to experience stresscracking and other dimensional product variations. Nevertheless, withproper controls in place, processing efficiency and consistency in theoutput of the corrugator can typically be maintained.

However, altering the material make-up of the polymer melt for the innerliner wall (e.g., by introducing an additive to the polymer melt toproduce a blended material having enhanced elastomeric properties) cansignificantly affect the processing start-up and output efficiency ofthe corrugator. LLDPE tends to have a lower melt strength, thusrequiring adjustments in the extrusion process. With LLDPE, it may benecessary to properly taper the corrugator cooling mandrel to offsetmaterial melt strength issues. TPEs, on the other hand, have mechanicalproperties that generally prove to be more workable as an additive tothe inner wall material and tend to improve the processing of coilabledual wall pipe during the manufacturing process. Nevertheless, the useof such additives will change the material composition of the innerwall, and can and will inevitably lead to variations in the extrusionprocess, thus possibly affecting the processing start-up and outputefficiency of the corrugator.

In order to minimize potential disruptions in the extrusion process andalleviate concerns as to the improper formation of the inner wall ofextruded corrugated dual wall pipe, an inner wall inspection system andmethod has been developed which utilizes automated laser technology toscan the surface of the inner wall for defects. With reference to FIG.4, it can be seen that, as the improved coilable dual wall pipe 101 withthe enhanced flexible inner wall 105 exits the corrugator (not shown),it telescopes over a three-dimensional (3-D) laser imager 201. The laserimager 201 scans the entire annular surface of the inner wall 105 of thedual wall pipe 101 for defects as it is being processed and reports anynoted defects to the corrugator operator.

The laser imager 201 must be capable of scanning 360° within the pipe.Since a standard laser line pattern generator is only capable ofemitting light as a 2-dimensional triangular wedge, it would requirealignment of multiple lasers for 360° coverage, making it virtuallyimpossible to maintain precise alignment. Therefore, in order to inspectthe entire inner annular surface 105 of the dual wall pipe 101, imager201 utilizes a conical laser pattern generator 203. As shown in FIG. 4,the 3-dimensional, conical shape 205 of the beam generator 203 casts athin circle against the inner wall 105 of the dual wall pipe 101,allowing for complete coverage of the interior pipe surface, withoutrequiring alignment of multiple generators.

For purposes of recording inspection results, the imager 201incorporates a three (3) camera configuration, with each camera 207spaced at 120° intervals, allowing for 40° overlap at the image edges.Each camera 207 is pitched 45° down, relative to the laser cone and pipedirection of travel. This places the center of the camera sensor inalignment with one of the three segments of the laser cone against theinner wall 105 of the dual wall pipe 101. With this cameraconfiguration, comprehensive 3-D coverage of the inner wall 105 may berecorded and analyzed for manufacturing defects as the dual wall pipe101 is being manufactured, thus ensuring structural integrity of themanufactured pipe.

With reference to FIGS. 5 & 6, recorded comparative imagery results froman inner wall liner 105 with and without defects is shown graphically.Each graph represents a 2D height map and cross section profile of aninner liner wall 105 taken from a single camera 207, using the conicallaser projection 205. FIG. 5 shows notable defects in the inner wallsurface at points 209 and 211. By contrast, the constant curve 213 shownin FIG. 6 represents a smooth inner liner wall with no surface defects.Each camera 207 records a 120° minimum section of the interior of thedual wall pipe 101, ensuring complete recorded coverage. With theforgoing inspection system and method, quality control as to thestructural integrity of the inner wall 105 of the coilable dual wallpipe 101 is assured.

The disclosure herein is intended to be merely exemplary in nature and,thus, variations that do not depart from the gist of the disclosure areintended to be within the scope of the disclosure. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention herein, which comprises the matter shown and described hereinand set forth in the appended claims.

1. Coilable dual wall corrugated plastic pipe, comprising: (a) anelongated section of hollow, generally cylindrical plastic pipe with alongitudinal axis, said pipe having an inner wall with a cylindricalsubstantially smooth inner surface and having a generally cylindricalcorrugated outer wall with spaced apart successive annular peaks andannular valley defining portions therebetween, said inner wall beingconnected to said corrugated outer wall at said annular valley definingportions thereof; (b) said outer corrugated wall being formed of athermoplastic material; (c) said inner wall being formed of a materialblend comprising a thermoplastic material and a thermoplastic elastomer;and (d) said material blend of said inner wall having a greater straincapacity and lower modulus of elasticity than said material of saidcorrugated outer wall, so as to provide sufficient flexibility andresiliency to facilitate bending and coiling of said pipe duringtransport and installation.
 2. The coilable dual wall pipe set forth inclaim 1, wherein said material blend of said inner wall is sufficientlyflexible and resilient to facilitate bending of said pipe without damageto a minimum bend radius which is approximately one-half the pipenominal diameter.
 3. The coilable dual wall pipe set forth in claim 2,wherein said bend radius of said pipe falls within a target range ofapproximately 0.5 to 4.0 times the pipe nominal diameter.
 4. Thecoilable dual wall pipe set forth in claim 1, wherein said materialblend of said inner wall is composed of less than about 40%thermoplastic elastomer by weight.
 5. The coilable dual wall pipe setforth in claim 1, wherein said outer corrugated wall is formed of amember of a material group including high density polyethylene,polyvinylchloride and polypropylene.
 6. The coilable dual wall pipe setforth in claim 1, wherein said material blend of said inner wall has ashort term modulus of elasticity below approximately 80,000 psi.
 7. Thecoilable dual wall pipe set forth in claim 1, wherein said inner wallforms a substantially smooth inner surface having a mannings coefficientless than 0.021.
 8. The coilable dual wall pipe set forth in claim 1,wherein said inner wall increases the axial strength of said pipe intension to an amount greater than corrugated single wall high densitypolyethylene pipe of similar diameter.
 9. The coilable dual wall pipeset forth in claim 1, wherein said inner wall increases the axialstrength in compression to an amount sufficient to assemble a bell andspigot compression joint.
 10. The coilable dual wall pipe set forth inclaim 1, wherein said material of said outer corrugated wall is a memberof a material group including high density polyethylene,polyvinylchloride and polypropylene, and said inner smooth wall isformed of the same said material as said outer corrugated wall blendedwith a thermoplastic elastomer.
 11. The coilable dual wall pipe setforth in claim 1, wherein said material of said outer corrugated wall iscomposed primarily of high density polyethylene, and said material ofsaid smooth inner wall is composed of a blend of high densitypolyethylene and a thermoplastic elastomer.
 12. The coilable dual wallpipe set forth in claim 1, wherein said material blend from which saidinner wall is formed has enhanced mechanical properties relative tolinear low density polyethylene that facilitate improved processing ofsaid pipe during manufacturing.
 13. Coilable dual wall corrugatedplastic pipe, comprising: (a) an elongated section of hollow, generallycylindrical plastic pipe with a longitudinal axis, said pipe having aninner wall with a cylindrical substantially smooth inner surface andhaving a generally cylindrical corrugated outer wall with spaced apartsuccessive annular peaks and annular valley defining portionstherebetween, said inner wall being connected to said corrugated outerwall at said annular valley defining portions thereof; (b) said outercorrugated wall being formed of a member of a material group includinghigh density polyethylene, polyvinylchloride and polypropylene; and (c)said inner wall being formed of a material blend comprising the samesaid material from which said outer corrugated wall is formed, and athermoplastic elastomer.
 14. The coilable dual wall pipe set forth inclaim 13, wherein said material blend of said inner wall has a greaterstrain capacity and lower modulus of elasticity than said material fromwhich said corrugated outer wall is formed, so as to provide sufficientflexibility and resiliency to facilitate bending and coiling of saidpipe during transport and installation.
 15. The coilable dual wall pipeset forth in claim 14, wherein said material blend of said inner wallhas a short term modulus of elasticity below approximately 80,000 psi.16. The coilable dual wall pipe set forth in claim 13, wherein saidmaterial blend of said inner wall is composed of less than about 40%thermoplastic elastomer by weight.
 17. The coilable dual wall pipe setforth in claim 13, wherein said plastic pipe is sufficiently flexiblelongitudinally to facilitate bending without damage to a minimum bendradius which is approximately one-half the pipe nominal diameter. 18.The coilable dual wall pipe set forth in claim 17, wherein said bendradius of said pipe falls within a target range of approximately 0.5 to4.0 times the pipe nominal diameter.
 19. The coilable dual wall pipe setforth in claim 13, wherein said material blend from which said innerwall is formed has enhanced mechanical properties relative to linear lowdensity polyethylene that facilitate improved processing of said pipeduring manufacturing.
 20. The coilable dual wall pipe set forth in claim13, wherein said inner wall forms a substantially smooth inner surfacehaving a mannings coefficient less than 0.021.
 21. The coilable dualwall pipe set forth in claim 13, wherein said inner wall increases theaxial strength of said pipe in tension to an amount greater thancorrugated single wall high density polyethylene pipe of similardiameter.
 22. The coilable dual wall pipe set forth in claim 13, whereinsaid inner wall increases the axial strength in compression to an amountsufficient to assemble a bell and spigot compression joint.
 23. Coilabledual wall corrugated plastic pipe, comprising: (a) an elongated sectionof hollow, generally cylindrical plastic pipe with a longitudinal axis,said pipe having an inner wall with a cylindrical substantially smoothinner surface and having a generally cylindrical corrugated outer wallwith spaced apart successive annular peaks and annular valley definingportions therebetween, said inner wall being connected to saidcorrugated outer wall at said annular valley defining portions thereof;(b) said outer corrugated wall being formed of a thermoplastic material;(c) said inner wall being formed of a material comprising a linear lowdensity polyethylene material having a greater flexibility than saidcorrugated outer wall, so as to provide sufficient flexibility andresiliency to facilitate bending and coiling of said pipe duringtransport and installation.
 24. The coilable dual wall pipe set forth inclaim 23, wherein said material of said inner wall is sufficientlyflexible and resilient to facilitate bending of said pipe without damageto a minimum bend radius which is approximately one-half the pipenominal diameter.
 25. The coilable dual wall pipe set forth in claim 23,wherein said bend radius of said pipe falls within a target range ofapproximately 0.5 to 4.0 times the pipe nominal diameter.
 26. Thecoilable dual wall pipe set forth in claim 23, wherein said material ofsaid inner wall is a material blend composed primarily of linear lowdensity polyethylene and a high density polyethylene or thermoplasticelastomer.
 27. The coilable dual wall pipe set forth in claim 23,wherein said outer corrugated wall is formed of a member of a materialgroup including high density polyethylene, polyvinylchloride andpolypropylene.
 28. The coilable dual wall pipe set forth in claim 23,wherein said material of said inner wall has a short term modulus ofelasticity below approximately 80,000 psi.
 29. The coilable dual wallpipe set forth in claim 23, wherein said inner wall forms asubstantially smooth inner surface having a Mannings coefficient lessthan 0.021.
 30. The coilable dual wall pipe set forth in claim 23,wherein said inner wall increases the axial strength in tension to anamount greater than corrugated single wall high density polyethylenepipe of similar diameter.
 31. The coilable dual wall pipe set forth inclaim 23, wherein said inner wall increases the axial strength incompression to an amount sufficient to assemble a bell and spigotcompression joint. 32-35. (canceled)